INVERTEBRATES HAVE LONG BEEN AN INVALUABLE SOURCE OF MODELS FOR THE FUNCTIONAL ANALYSIS OF THE CENTRAL NERVOUS SYSTEM. EACH GROUP HAS HAD ITS STRENGTHS, THE MOLLUSCS BEING PARTICULARLY RELEVANT FOR STUDIES OF CELLULAR PHYSIOLOGY AND BEHAVIOR. IN PARTICULAR THE NUDIBRANCH HERMISSENDA CRASSICORNIS HAS FOUND ITS NICHE IN THE STUDY OF LEARNING AND MEMORY, NOTABLY ASSOCIATIVE OR PAVLOVIAN CONDITIONING. OUR KNOWLEDGE OF THE LATTER SYSTEM IS NOW EXTENSIVE BUT HAS BEEN HAMPERED BY AN INABILITY TO CULTURE THE ISOLATED NEURONS OR CIRCUIT COMPONENTS OF THE CONDITIONING REFLEX. THE FAILURE TO GET HERMISSENDA NEURONS TO GROW IN CULTURE IS SURPRISING, BECAUSE OTHER MOLLUSCANS HAVE BEEN CULTURED SUCCESSFULLY AND RELATIVELY EASILY [E.G., APLYSIA; HELISOMA], AND SOME SYSTEMS HAVE BEEN USED TO RECONSTRUCT CIRCUITS IN VITRO. THIS STUDY WAS CONCERNED WITH A RELIABLE TECHNIQUE FOR THE CULTURE OF NEURONS FROM HERMISSENDA. PRELIMINARY DATA GIVES THE EFFECT OF ADHESION MOLECULES AND THE ELECTROPHYSIOLOGY OF ISOLATED NEURONS. CELLS OF PARTICULAR INTEREST IN THIS STUDY INVOLVED THOSE IN THE PHOTOTACTIC CIRCUITRY AND ASSOCIATIVE CONDITIONING PATHWAY, FOR EXAMPLE, THE LEFT PEDAL NEURON (LP1), A PARTICULARLY LARGE MOTORNEURON IN THIS CIRCUIT. THE AVAILABILITY OF THESE CULTURED NEURONS WILL PROVIDE AN APPROPRIATE MODEL FOR THE STUDY OF THE ACTION OF LEAD AND OTHER HEAVY METALS IN CONJUNCTION WITH THE APPLICATION OF PROBE TECHNOLOGY. THIS STUDY IS SUPPORTED BY A GRANT FROM THE MBL DIRECTOR AND THE PROVISION OF EQUIPMENT FROM THE LABORATORY OF DR. D. ALKON, NINDS, BETHESDA.